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Old Jan 01, 2011, 02:47 AM
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Australia, ACT, Kambah
Joined Feb 2001
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Motor power ratings

I've found people often getting mixed up by power ratings. The problem seems to me that people see a rated power for an electric motor and not unreasonably assume that that is the max it will deliver, like horsepower for a car engine. But it's not that way at all - the motor will happily destroy itself trying to turn whatever prop is attached at a speed depending on the Kv of the motor and the voltage of the battery. If the prop is too big for Kv times volts or the Kv is too high for that prop at your voltage, then in trying to turn it the motor is going to want to draw a lot of current from the battery - maybe way over the rated power if the battery can deliver the current and the ESC doesn't fail first.

So what's the use of the power rating? The manufacturer is trying to tell us that the motor at the rated max input power is capable of operating at some reasonable (but almost never specified) efficiency level that won't generate so much waste heat that you cook the motor. For cheap brushed motors, that efficiency might be as low as 50%. For cheap outrunners, maybe 60-66%. In a roundabout way, the power rating is almost more about how much wasted heat the motor can cope with without getting so hot that magnets fail, etc.

So given that the motor won't actually limit itself to the stated power rating, it's up to us as the power system designer to set things up so that we provide enough power for desired performance and that the motor won't want to absorb more than its safe power level (and not exceed the rating of the ESC you chose, and not overstress your battery). We do that by ensuring that the combination of prop size, motor Kv and battery voltage will work together - you can't treat each one in isolation.

Other replies have given tried and true examples of what does work. For the APC 6x4E I use an 1800Kv motor for 3S (mega 16/15/5 and Keda 2040-31L) and a 2400Kv for 2S (Keda 2040-24L). On an EasyStar I also use the stock Mulitplex 5x4 prop on the 2400Kv motor on 2S and while less thrust than the 6x4 it flies fine and almost forever!

A really useful whatif tool to find workable combinations that doesn't rely on the detailed specs of motors is Webocalc accessed from here: http://flbeagle.rchomepage.com/. It's got a great tutorial, too.
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